This invention relates to planar optical waveguides and, in particular, to planar optical waveguides which include bends.
Optical waveguides exist in two configurations, namely fibre and planar. The planar configuration is convenient for the processing of optical signals and the term xe2x80x9cplanarxe2x80x9d is used because the path regions are located in an essentially two-dimensional space. Typically the path regions are formed of an amorphous material and they are enclosed in a matrix of one or more different amorphous materials ideally having the same refractive index as one another. The refractive index of the matrix is less than the refractive index of the material forming the path regions. The difference between the two refractive indices is often represented by xcex94n and for the condition for effective guidance with low attenuation is usually
xcex94n=0.01(approximately).
The amorphous materials are preferably glass, e.g. silica based glass or some polymeric material such as an organic plastics material. Silica doped with germania is particularly suitable for the path regions. In the case of the matrix pure silica or silica containing processing aids such as oxides of phosphorus and/or boron are particularly suitable. (Pure silica has a. refractive index of 1.446 and this is a convenient refractive index for the whole of the matrix. Germania increases the refractive index of a silica glass.) It is of course possible to use pure or substantially un-doped silica for the path region with index-depressed doped silica as the cladding. As an alternative to the use of amorphous materials, it is known to use crystalline materials, such as single-crystal silicon (typically epitaxially grown) as the path region. With silicon, the path region is typically surrounded by a lower index amorphous material such as silica or doped silica. It is known however to have both the path and cladding regions formed of single crystal semiconductor materials (again, usually epitaxially grown). Although the invention is described in this application with reference to the use of amorphous materials, which are preferred, the invention has application to waveguides formed of these other material types and no limitation is intended to the use of amorphous materials.
European Patent Specification EP-A-0 583 903 describes arrayed waveguide gratings which consist of an array of curved planar waveguides of different lengths. The waveguides are closely spaced at their ends and widely spaced and strongly curved in the central region. End losses andlor phase errors are reduced at the bends by making the width of the planar waveguide sufficiently large to cause the fundamental mode of the optical signal to be displaced away from the inner edge of the curve. The fundamental mode effectly propagates in the vicinity of the outer edge of the bend and its propagation constant becomes effectively independent of the width of the waveguide. As a result, width variations of the planar waveguide do not contribute to loss and the bend radius can be reduced so that only the fundamental mode is guided round the bend without suffering considerable loss.
Although, as mentioned above, planar waveguiding structures are not fibre, the term xe2x80x9ccorexe2x80x9d is often used to denote the path regions and the matrix in which the cores are embedded is often called the xe2x80x9ccladdingxe2x80x9d.
The condition stated above is appropriate for most of a waveguide but this invention relates to special portions where different considerations apply. According to this invention a planar waveguiding device includes regions wherein a segment of core is located adjacent to a groove or between two grooves. Preferably the groove or grooves extend above and below said segment of core. It is desirable that the evanescent fields of signals travelling in the core penetrate into the groove.
The maximum extent of the evanescent fields outside the core is usually less than 1 xcexcm and therefore any coating between the core and the groove should be less than 500 nm. Preferably there is a direct interface between the core and the groove. Localised heating of cores offers one way of causing localised changes of refractive index, e.g. for Max Zehnder devices. A heating element can be located on top of the core adjacent to one or two grooves. The grooves restrict the transmission of heat.
In some applications material may be located in the groove, e.g. for use as a sensor or for testing the material in the groove. In these applications the material is placed in the groove after the device has been made, e.g. material is placed in the groove and , if necessary, replaced in accordance with requirements.
Usually the purpose of the groove is to provide a very low refractive index adjacent to the core, i.e. to make xcex94n as big as possible. The lowest refractive index, namely 1, is provided by an empty groove (i.e. vacuum) but most gases also have a refractive index substantially equal to one. xe2x80x9cEmptyxe2x80x9d grooves as described above are particularly valuable where cores pass round bends. This is a preferred embodiment of the invention and it will be described in greater detail below.
A high proportion of the cores consists of straight lines but possible uses are severely limited if the cores consist only of straight lines and, in general, signal processing is not possible in planar devices wherein the cores consist only of straight lines. Many planar devices include multiplexers and/or demultiplexers and curves are needed to form these. Curves are also needed if it is desired to create a serpentine path in order to increase its length, e.g. for a laser. Complicated devices, such as arrayed waveguide gratings (AWG), require many bends.
In many devices the radius of curvature of the bend is a critical parameter in determining the overall size of the device. For example, a small radius of curvature will place waveguide segments close together whereas a large radius of curvature will cause the segments to be more widely separated. In order to provide more processing capability on the same size of wafer it is desirable to make the devices as small as possible and, since the radius of curvature is a critical parameter, it is desirable to make the radius of curvature as small as possible. In some cases, the spacing of waveguides on a wafer is determined by external constraints and it may be necessary to use a small radius of curvature in order to conform to the external constraints.
It will be appreciated that a curved path may be a circle or a segment of a circle and in such a case the radius of curvature of the path is constant, i.e. it is equal to the radius of the circle. If a curved path is not circular it will still have a radius of curvature but this radius will vary from point to point along the curve. Nevertheless, it is still true that a small radius of curvature will favour closer packing of devices. It is usually convenient to measure the radius of curvature to the centre of the core but there will be significant differences between the inside and the outside of the curve.
The guidance of optical radiation round shallow bends, e.g. with radii of curvature of 5 mm or more does not cause problems but sharp bends, e.g. with radii of curvature below about 2 mm, can cause noticeable degradation of performance.
These problems can become severe when it is desired to use even smaller radii of curvature, e.g. less than 500 xcexcm.
According to a preferred embodiment of this invention, a planar waveguiding device comprises a core having a bend with an inner radius of curvature and an outer radius said inner radius of curvature being less than 2 mm wherein xe2x80x9cemptyxe2x80x9d grooves are located adjacent to both said inner and said outer radii of curvature, said grooves preferably having an interface with the core and extending both above and below the core. Since the grooves are prepared by etching they will normally extend to the surface of the device but it is desirable to continue the etching below the bottom of the core in order to improve the guidance. It has been stated that the grooves are xe2x80x9cemptyxe2x80x9d. Conveniently, the grooves are allowed to contain whatever atmosphere is present where the device is used. In most cases, the atmosphere will be air but, in space there would be a vacuum. The refractive index in the groove is substantially equal to one because this is the refractive index of a vacuum and virtually all gasses have a refractive index equal to one.
In one aspect, this invention is concerned with the problem of loss of guidance at bends which may result in the radiation escaping from the core. The severity of this problem is strongly related to the radius of curvature of the bend and the smaller the radius of curvature the worse the problem. Where the radius of curvature is above 5 mm there is no problem but there is a substantial problem when the radius of curvature is 2 mm or less. The problem gets even worse at smaller radii of curvature, e.g. below 500 xcexcm. The location of grooves will give usefully low attenuation at radii of curvature down to about 50 xcexcn. It will be appreciated that some waveguiding structures will include a plurality of bends. There would be no advantage in providing grooves adjacent to curves with radii of curvature greater than 5 mm and it is highly desirable that all bends with radii of curvature less than 2 mm, and especially less than 500 xcexcm, are provided with grooves in accordance with the invention.
The electric and magnetic fields associated with light propagating in the cores extend outside the cores and, ideally, the groove should be so located and sufficiently wide that these fields are contained entirely in the grooves. For wavelengths of the order 1.5 xcexcm the fields extend for about 1 xcexcm beyond the core. For most purposes, grooves which are 30 xcexcm wide will be sufficient. There is no objection to using greater widths where these are convenient and compatible with the overall structure.
Waveguiding devices in accordance with the invention can be manufactured using conventional fabrication techniques. For example, it is convenient to deposit a sequence of glass layers by flame hydrolysis using conventional photolithography to produce path regions and grooves. In order that there is an interface between the groove and the core, it is appropriate to etch the core to extend beyond the boundaries of the curve and to remove core material when the groove is etched. Reactive ion etching is particularly suitable for producing the grooves because this technique is inherently monodirectional and it produces grooves with vertical sides.